Multistage amplifier for utilizing secondary emission



' H. STRVUBIG Jan. 28, 1941.

MULTISTAGE AMPLIFIER FOR UTILIZING SECONDARY EMISSION Filed Feb. 9, 1957 2 Sheets-Sheet l swam tom Jan; 28, 1941. H. STRUBIG 2,230,124

MULTISTAGE AMPLIFIER FOR UTILIZING SECONDARY EMISSION I Filed Feb. 9, 1937 2 Sheets-Sheet 2 Patented Jan. 28, 1941 PATENT OFFICE MULTISTAGE AMPLIFIER FOR UTILIZING SECONDARY EMISSION Heinrich Striibig, Teltow, near Berlin, Germany,

assignor to the firm Fernseh Aktien-Gesellschaft, Zehlendorf, near Berlin, Germany Application February 9, 1937, Serial No. 124,937 In Germany February 10, 1936 2 Claims.

This invention is related to an amplifier utilizing secondary emission with several cascaded stages of amplification. In these amplifiers a beam of electrons impacts an emitting electrode 5 with such velocity that the ratio of secondary electrons to primary electrons is greater than unity. The secondary electrons are again accelerated and the process of secondary electron amplification is repeated in further amplification stages,

The anode of each amplifier stage represents the cathode of the following stage.

In amplifier tubes of this type, as known so far, the disadvantage existed that the accelerating fields in the different amplification stages were not clearly separated. Of the lines of force which ended on every emitting electrode, some had the tendency to accelerate the electrons towards the electrode, whereas others had the tendency to draw the electrons away from this electrode. As a consequence thereof, many electrons passed the emitting electrode without impacting, and thereby did not contribute to the amplification by generation of secondaries. Furthermore, the generation of a space charge was hereby favored. These phenomena impaired the efiiciency of the amplifier. According to the invention, the accelerating fields between two subsequent amplification stages are separated by a permeable auxiliary electrode disposed between two consecutive emitting electrodes. The auxiliary electrode is held at about the same potential as the second of the above mentioned emitting electrodes. Preferably, a grid structure is used as an auxiliary electrode and is directly connected with the following emitting electrode. The auxiliary electrode may also consist of a foil, or a sieve, or a combination of these elements. The lines of force of the electric field in one stage run from the foregoing electrode to the grid, so that the lines of force of the following amplification stage have the full area of the next emitting electrode at their disposal. The electrons are accelerated by the grid to a velocity suitable to liberate secondaries and fly with this velocity 5 through the grid toward the emitting electrode. With a suitable construction of the emitting and auxiliary electrodes, the influence of the field of the following amplification stage on the electrons is very slight because of the high velocity of the latter. Thus use is made of nearly all electrons traveling through the grid for secondary emission, and all liberated secondary electrons are accelerated towards the following emitting electrode. Only a small amount of electrons will strike the grid. These electrons may be also made useful for the amplification by coating the grid with a strongly secondary emissive material as, for instance, caesium.

The further improvement of the efficiency is 5 obtained by spacing a further auxiliary electrode, preferably a grid, between the emitting electrode and the following auxiliary electrode. This further auxiliary electrode will be then held at a potential between that of the emitting electrode 10 and a first auxiliary electrode. The arrangement and the intermediate potential are preferably chosen in such a manner that the acceleration of the electrons occurs substantially in a homogeneous field between .the two auxiliary 15 grids. If the potential of the further auxiliary grid (hereafter referred to as the intermediate grid) is suitably chosen, all electrons of the foregoing stage must strike the emitting electrode. At the same time the voltage is chosen in such a manner that all secondary electrons are drawn away from the emitting electrode by the intermediate grid. Usually the voltage between the intermediate grid and the emitting electrode will be a fraction of the total of the acceleration volt- 25 age.

Figure 1 shows a cross-section of an amplifier utilizing secondary emission of the known kind.

Figures 2, 3 and 4 show modifications of amplifiers with auxiliary electrodes, according to the 30 invention.

In Figure 1, I, 2, 3, 4, 5 and 6 are emitting electrodes disposed in the interior of a highly evacuated amplifier tube l5. The potential difference between subsequent electrodes is constant, for instance, if the potential of the second electrode is 400 volts in respect to that of the first electrode, every following electrode will be 400 volts higher than the foregoing electrode. Electrons are liberated on the cathode I, for instance, 40 by photo-emission, and are first accelerated towards the electrode 2, where they liberate secondaries which are accelerated toward the electrode 3. The process of secondary emission is then repeated. The same happens on the electrodes 4, 5 and 6. Further emitting electrodes may follow the electrode 6, but are not shown in the drawings. The end electrode, which is also not shown, serves as a collector.

From the course of the indicated lines of force, 50 it may be seen that lines of force of different directions end on each of the electrodes 2, 3, 4,

5 and 6. Furthermore, it may be seen that quite a few lines of force lead immediately to the electrode following the next, thus leaving one elecings.

trode out. An electron which may, for instance, leave in the middle of an emitting electrode, will first follow the lines of force and will then travel with comparatively high velocity parallel to the electrodes without striking. Thereby, the electron is lost for the amplification by secondary emission.

Figure 2 shows how the arrangement shown in Figure 1 is provided with grids 1, 8, 9, l0 and II according to the invention, which grids are at the same potential as the following emitting electrodes. Again lines of force are indicated from the course of which may be seen that all lines of force of one stage now end on the inserted grid, so that the total area of the emitting electrode is penetrated by lines of force of the following acceleration stage.

Figure 3 shows another modification in which a further grid is disposed between the auxiliary electrode and the emitting electrode. Voltages, increasing in steps of 400 volts, are applied to the electrodes 2, '3, 4 and 5. Grids B, 9 and I0 correspond to those marked by the same numbers in Figure 2. Between each of the grids 8, 9 and I0 and the foregoing electrode, further intermediate grids l2, l3 and M .are arranged, which are held at .a potential of about 50 volts above the foregoing electrode. The shaping of the electric field is schematically shown in the draw- The acceleration of the electrons occurs substantially between the two grids. The voltage of the grids 12, I3 and [4 in respect to the electrodes 2, 3 and 4, is chosen in such a manner that no noticeable influence is exerted on electrons of high velocity flying through the grids 1, 8 and 9, but that all secondaries emitted by the plates 2, 3 and 4, which at first have only a low velocity, are accelerated towards the grids l2, l3 and I4, and thus guided to the next amplification stage.

Figure 4 shows a further modification, in which the surface of the emitting electrodes increases from stage to stage, so that the current density per unit of area stays approximately constant. In the vacuum receptacle I5 is the thermionic cathode it, which emits the primary electrons, the number of which is controlled by the grid H. The electrons impact .a cone shaped first emitting electrode I8 and liberate secondaries which leave the cone l8 substantially perpendicular to the axis of the cone l8, and arrive at the .lar to the electrodes I9 and 20.

secondary emission repeats itself at the electrodes 2|, 22 and 23, the construction of which is simi- The numbers I, .8, 9, l0 and II again indicate grids which are at the same potential as the electrodes I9, 20, 21, 22 and 2'3 respectively. The electrons from the electrode 23 are collected by the collector 2'4.

"Iclaim:

1. An electron multiplier comprising an envelope having therein electrodes, each of said electrodes having an electron emissive surface of area diiferent from that of the other electrode surfaces, means supporting said electrodes with said emissive surfaces arranged in the order of their progressively increasing areas and positioned substantially parallel to each other to be successively impacted by an electron streamemitted .by an initial one of said surfaces, the

arrangement of said surfaces normally producing undesirable electrostatic fields between alternate ones of said surfaces which detrimentally affect the movement of said electron stream between successive ones of .said surfaces, and electron permeable means positioned in the path of said electron stream between each pair of said surfaces at an angle with respect to the electron path between said surfaces for reducing the :detrimental effects on the movement of said electron-stream .of said electrostatic-field.

2. An electron multiplier-comprising an envelope having therein a. seriesofsecondarily-emissive electrodes, and means for developing electrostatic fields only betweensuccessive electrodes in such series and preventing the development of electrostatic fields between electrodes other than successive electrodes, said means comprising a plurality of electron permeable grids disposed between .successive'electrodes, one ofsaid gridsbeing directly connected to the .electrode of the higher stage and the other of said grids being free for application thereto of an electromotive force slightly above that 'to be applied to the electrode of the lower stage.

HEINRICH .STRU'BIG. 

